1. Field of the Invention
The present invention relates to a power amplifier applied to a dual mode mobile radio apparatus and to a mobile radio apparatus with the power amplifier.
2. Description of the Related Art
In mobile radio communication systems such as a cellular telephone system, a portable telephone system, and a cordless telephone system, an analog communication system is conventionally applied in consideration of wave environment, circuit size, frequency utilization efficiency, and the like. In recent years, however, a shift from the analog communication system to a digital communication system has been examined. When the shift from the analog communication system to the digital communication system is to be performed, in a transition period of the shift, it is expected that a system (which employs a dual mode radio communication system) including analog and digital communication systems is used for convenience to a user of a conventional analog communication system.
In a digital communication system, a linear modulation system, e.g., a quadrature amplitude modulation system, having a high frequency-utilization-efficiency, has received a great deal of attention.
In an analog communication system, a nonlinear modulation system such as a narrow band FM system is generally employed. For this reason, in a transmitter of a conventional mobile radio apparatus, a saturated amplifier having low current consumption and a high power-added-efficiency (a ratio of DC power consumption to a radio frequency output power) is used as a power amplifier. Therefore, when a carrier linear-modulated as described above is amplified by this conventional power amplifier, distortion occurs in the carrier due to nonlinearity of the power amplifier, and transmission characteristic is considerably degraded.
In order to avoid this disadvantage, it is understood that a linear amplifier for performing class A amplification or class AB amplification is used as the power amplifier. In this linear amplifier, both of a nonlinear-modulated carrier and a linear-modulated carrier can be amplified without distortion.
However, the linear amplifier has high current consumption, therefore, low power-added-efficiency. Since a mobile radio apparatus is often driven by a battery, the increase in current consumption poses a serious problem. In this case, although total current consumption can be reduced by applying a time-division multiple access (TDMA) system in a digital communication system, an increase in current consumption cannot be easily prevented in an analog communication system.
In order to avoid these various disadvantages, it is understood that a saturated amplifier is used as a power amplifier and that distortion occurring in this saturated amplifier is compensated by adding a compensation circuit. However, this addition of the compensation circuit increases the circuit size and the size of the apparatus. Therefore, a serious disadvantage, i.e., low portability, occurs in a mobile radio apparatus. In addition, the compensation circuit has various disadvantages such as high power consumption, a high cost, and a demand of accurate adjustment in manufacturing steps.
As described above, according to a conventional power amplifier, it is difficult to preferably amplify both of a nonlinear-modulated carrier and a linear-modulated carrier. When both of the nonlinear-modulated carrier and the linear-modulated carrier are preferably amplified, the current consumption of an apparatus must be increased, or the size and cost of the apparatus must be increased.
Therefore, the above conventional mobile radio apparatus to which a power amplifier is applied has high current consumption or a large scale and a high cost.